Due to an ever dwindling supply of standard dry raw materials the problem of processing materials available in a naturally moist condition (with a moisture content of 25 to 28%) has nowadays gained tremendous significance. Quite frequently such materials are found to be contaminated with extraneous inclusions, which makes their processing with the conventional techniques next to impossible, not to mention obtaining therefrom a quality product. For instance, loose wet chalk usable in the production of lime and cement as well as clays employed in the manufacture of bricks and ceramic tiles are known to contain appreciable amounts of solid inclusions whose particles may broadly vary in size. The removal of extraneous impurities from such materials can be performed only by way of converting them to the form of suspensions exhibiting an adequate level of fluidity. Commonly such suspensions show a moisture content of 37.0 to 42.0%.
It might be well to point out that the problem of suspension processing is currently of considerable practical interest also in connection with the exigence of processing waste materials of diverse manufacturing sectors which may contaminate the environment. The processing of such waste affording to obtain products used in a number of production processes makes the technology practically waste-free, which enables to achieve a reduction in manufacturing costs. Thus, for example, there may be processed sugar production filtration sediments for the purpose of producing therefrom lime and carbon dioxide, kraft-pulp making and chemical water purification waste for producing lime and mineral fertilizers, metallurgical process waste, etc.
In the overwhelming majority of cases for processing suspensions use is now made of spray-drying methods allowing to significantly intensify drying processes through the provision of a developed surface of the material being dried and thermal treatment conducted in a suspended state. However, such drying results in the material having a heterogeneous granulometric composition and a large amount of fine fractions. At the same time it is frequently required that the treated material possess a granule size in the range of 250 to 800 microns and possibly be monofractional. Thus, for example, in cement production, in which dried granules are subjected to high-temperature heating and decarbonization, it is required that the granules have a size of 400 to 800 microns. In the production of mineral fertilizers it is also required that the material do not include pulverized fractions (with a size of less than 20 microns) and the granules have a size of 200 to 400 microns.
As is known, material granulation is achievable only through the use of special treatment methods and special devices. At the same time it would be expedient to discard the use of any specialized granulating devices whatsoever and to accomplish granulation in the process of drying so as to obtain a granulated product at the same stage of the production process coinciding with that of drying.
It should be noted that apart from drying of the material some production processes call for its high-temperature treatment, which may exert a marked impact on the effectuation of the subsequent procedure of processing a thermally treated product.
Thus, for example, preliminary decarbonization of cement-raw granules enables to raize the output of a cement clinker producing unit 2.5 to 3 times. The granulated materials can be also processed more qualitatively than polydisperse ones because the uniformity of granule sizes allows to provide for them more favorable thermodynamic treatment conditions.
It is known that drying of the material is effected by supplying a heat-carrier to the drying zone. The high-temperature treatment also calls for heating heat-carrier granules. Hence, it would be more economic and expedient to carry out both the procedure of thermal treatment and that of drying using one and the same heat-carrier.
It should be also emphasized that in some cases the treatment of a suspension requires that appropriate provision be made to accomplish drying with a capacity differing from the rated one in 5 to 10 times, while maintaining a constant specific fuel consumption rate. Thus, such requirements are imposed on the processing of sugar production waste when changing the type of raw material, for example, when using as raw materials sugar beet and raw sugar cane.
Accordingly, it has become necessary to evolve a method of and apparatus for producing granulated products from suspensions, which would permit to accomplish drying of the suspension with the production of a granulated product as well as its high temperature thermal treatment to give this product the desired qualities.
Known in the prior art is a method of producing a granulated product from suspensions (see U.S.S.R. Inventor's Certificate No. 393547, Int. Cl. F 26 B3/12, filed Aug. 10, 1973/, which comprises spraying the suspension by a flow of a heat-carrier, drying the same thereby, separating a pulverulent fraction from the dried material in an individual unit, heating this pulverulent fraction by an additional flow of a heat-carrier and supplying it in concentric parallel flows to the suspension spraying zone.
An apparatus for practicing the above-described method comprises a drying chamber having a plurality of concentric guides for a pulverulent fraction, mounted thereinside, a cyclone-powder separator, a powder heating device, a device for mixing powder with an additional heat-carrier flow, means for pneumatically conveying powder to the drying chamber.
The foregoing method of and apparatus for producing a granulated product is characterized by that the pulverulent fractions separated from the dried material are only those which have a size of less than 0.005 mm, while the dried material is not subjected to sizing. Hence, the dried material has a polydisperse composition. Furthermore, the method and apparatus described hereinabove do not enable to effect high-temperature thermal treatment of the granulated product, which essentially confines their range of application. It should be also noted that the method and apparatus involve much complexity in implementation since they include an objectionable multiplicity of operations and auxiliary devices. To such operations and auxiliary devices belong powder separation and powder separating cyclone, powder heating and powder heater, powder conveyance and powder conveying means, all of these operations being carried out at different stages, which makes the process unnecessarily protracted.
The above-described method and apparatus are also characterized by that heating of the dust by an additional heat-carrier flow brings about an increase in the consumption of fuel.
Known in the prior art is a method of producing a granulated material from suspension (see U.S.S.R. Inventor's Certificate No. 402726, Int. Cl. F 26 B 3/12, filed Aug. 16, 1971/, which comprises spray-drying the suspension, removing the dried material from a drying chamber and separating into fractions by multistage air sizing, injecting fine fractions of the dried material by a separate air flow to a hollow spray of the suspension.
An apparatus for practicing this method comprises a spray-type drier provided with a pneumatic nozzle for spraying material (suspension) being dried, a cyclone for separating dried material from waste gas, a multistage air sizer, a mixing chamber for mixing air with dust, a compressed air source, an ejecting nozzle for supplying dust to the hollow spray of suspension.
The above-described method and apparatus are characterized by that the procedure of segregating dried material into fractions is conducted in the form of a separate operation done in the self-contained structurally complex multistage sizer. Moreover, the supply of fine fractions of dried material to the hollow spray of suspension, i.e. to the spraying chamber, calls for a special device and compressed air energy. All this makes the method too complicated, adds to the duration of the process and results in the overly sophisticated design of the entire apparatus. It should be also noted that the foregoing method and apparatus exclude the possibility of effecting high-temperature thermal treatment of the granulated material, which makes them useful only for drying processes and thereby confines their application range.
Known in the prior art is a method of producing granulated materials comprising spraying a suspension by a rotating disc, drying the suspension, removing waste gas with a portion of the dried product, separating a portion of the dried product from the waste gas and supplying the aforesaid portion of the product to the rotating disc (see F.R.G. Pat. No. 2,201,111, Int. Cl. F 26 B 3/12, filed Jan. 11, 1972).
An apparatus for practicing this method comprises a drying chamber having a spraying rotating disc, means for supplying a suspension to the rotating disc, means for removing waste gas with a portion of the dried material, a cyclone for separating a dried product from the waste gas, said cyclone being mounted inside the drying chamber coaxially with the spraying rotating disc at a small distance therefrom, an exhaust pipe for removing waste gas connected to the cyclone and means for unloading a finished product from the lower portion of the drying chamber.
The above-described method of and apparatus for producing granulated materials are characterized by that to the hollow spray created by the spraying rotating disc are supplied unsized fractions of the dried material, which at a subsequent stage excludes the possibility of obtaining a monofractional finished product. Furthermore, the dried product is passed to the rotating disc by the flow of waste gas, as this takes place, the particles of the product are not heated and, consequently, a decrease in the intensity of heat and mass transfer process occurs. It should be noted that the supply of the dried product to the rotating disc is accomplished solely by meuns of air rarefaction provided in the rotation of the disc. As a result, such a method and apparatus are operable only with disc spraying and hardly applicable with other methods of suspension spraying. What is more, the abovedescribed method and apparatus are complicated, while the provision of the cyclone inside the drying chamber reduces its capacity, which causes a decrease in the efficiency of the method and apparatus and makes the maintenance of the latter difficult.
The above method and apparatus are also characterized by that they do not afford the effectuation of high-temperature thermal treatment of the dried granulated material, which confines their application range.
Known in the prior art is a method of producing granulated materials from a suspension (see French Pat. No. 2,266,129, Int. Cl. F 27 B 15/00; B01j 6/00; C01F 5/10, filed Oct. 24, 1975), which comprises spraying the suspension in a thermal treatment chamber, drying and thermally treating the same by the flow of a heat-carrier introduced to the chamber tangentially, withdrawing the thermally treated material from the chamber by means of an unloading device, a portion of the thermally treated material being withdrawn from the thermal treatment chamber together with the waste gas. This is followed by separating this portion from the flow of the waste gas in a separator arranged beyond the thermal treatment chamber and passing it to the zone located beneath the mounting zone of burners, or directly into the burners.
An apparatus for practicing the above method of producing granulated materials comprises a cylindrical thermal treatment chamber, suspension spraying means, burners mounted tangentially inside the lower portion of the chamber, a gas flue for withdrawing waste gas serving for the separation of material particles from the waste gas, a pipe line for returning this material to the zone located beneath the mounting zone of the burners or directly into the burners, and means for unloading thermally heated materials from the lower portion of the chamber.
However, the above-described method and apparatus do not enable to produce a thermally treated product having a homogeneous granulometric composition due to the absence of product granules sizing, which results in the polidisperse makeup of the finished material. Furthermore, the material separated from the waste gas is returned to the chamber, wherein it is also mixed with the remaining material subsequent to roasting. Consequently, this leads to an increase in the number of pulverulent fractions in the finished product, thereby increasing the polydisperse nature of the finished product. It should be also noted that the separation of the pulverulent fractions in an individual unit and the necessity of resupplying these fractions into the apparatus makes the method and design of the apparatus objectionally complicated. The prior art method and apparatus are also characterized by that they do not permit to effect treatment of the suspension with varying capacities (different from the rated capacity in 5 to 10 times) upon the retention of a constant specific fuel consumption rate. This is attributable to the fact that a decrease in the capacity and a reduction in the fuel consumption cause disturbances in the aerodynamic operating conditions of the apparatus (the amount of fuel decreases, while the dimensions of the apparatus remain unchanged). As a result, the application range of the method and apparatus is restricted.
Known in the prior art is a method of producing granulated materials from suspensions (see French Pat. No. 2080016, Int. Cl. P 26 B 5/00, filed 18, 1971) comprising spraying the suspension into the downcoming flow of a heat-carrier provided by tangentially mounted nozzles and drying it therein upon its moving downward in a direct current wth the heat-carrier, subsequently separating the dried material from the heat-carrier by means of creating another upcoming flow of air at the confluence area of the upcoming air flow and the downcoming heat-carrier flow, and unloading the material from the apparatus.
An apparatus for practicing the above method of producing granulated materials comprises a cylindrical drying chamber divided by a diaphragm into two compartments--the upper and lower one, means for spraying the suspension, means for introducing a heat-carrier and air mounted tangentially so as to form a downcoming heat-carrier flow in the upper portion of the chamber and an upcoming air flow in the lower one, and an unloading device mounted at the level of the location of the diaphragm.
The above-described method and apparatus are characterized by that the fine fractions of the material are mixed with the large ones at the unloading point. As a result, the dried material has a polydisperse composition. Furthermore, the method and apparatus are characterized by the complexity of carrying out the process of obtaining granulated materials as well as by the complexity of the apparatus design.
It should be also noted that the foregoing method and apparatus do not allow to produce granulated material with a varying capacity under a constant specific fuel consumption rate since the processing of the material according to such a method and in such an apparatus calls for the close observance of aerodynamic conditions which undergo disruption upon a decrease in the consumption of fuel. Consequently, the material processing with a small capacity brings about an increase in the specific consumption of fuel.
Known in the prior art is a method of an apparatus for producing granulated products from suspensions (see French Pat. No. 2,083,223. Int. Cl. F 26 B 3/00, filed Oct. 12, 1971, claims priority of July 7, 1970, Japanese Pat. No. 47 026, 1970), which is closely related in its technical essence to the subject of the present invention and therefore identified as prototypal.
The method under consideration comprises the steps of forming a bed of dried material solid particles at the bottom of a drying chamber defined by a vertical cylinder, blowing a heat-carrier into the foresaid bed through the lower portion thereof for the purpose of forming a "fluidized" bed of dried material solid particles, supplying the heat-carrier to the drying chamber in a downcoming vortex flow along the lateral walls of the drying chamber so as to provide a circulation of the dried material solid particles from the "fluidized" bed up to the axis of the drying chamber, spraying the suspension in the upper portion of the drying chamber during which the drops of the suspension collide with the fine particles of the dried material circulating from the "fluidized" bed with the formation of larger particles, drying these large particles in a direct current with the heat-carrier, withdrawing the largest particles of the dried material from the "fluidized" bed.
The "fluidized" bed of the solid particles of the dried material is interpreted in this case as the conversion of the bed of granular loose dried material to a pseudo-liquid under the effect of the flow of a fluidizing agent (i.e. heat-carrier) passing through the bed.
An apparatus for practicing the above-described method comprises a drying chamber designed in the form of a vertical cylinder provided with an opening for gas inlet in the upper portion of the drying chamber by means by which there is formed a spiral downcoming heat-carrier flow, suspension spraying means mounted inside the drying chamber, a perforated plate mounted in the lower portion of the chamber and means for blowing a heat-carrier through the perforated plate for the formation of a "fluidized" bed of dried material solid particles, an opening for waste gas outlet made in the upper portion of the drying chamber and means for withdrawing a finished product from the drying chamber.
The above-described method of and apparatus for producing granulated products are characterized by that in order to accomplish the process of granulation use is made of two heat-carrier flows, one of which is directed to the hollow spray of suspension, while the other to the "fluidized" bed of dried material solid particles. In this case the presence of the "fluidized" bed of dried material solid particles require precise proportioning of the amount of the suspension supplied and the amount of the material contained in the "fluidized" bed of solid particles inasmuch as upon an increase in the amount of the suspension dried the "fluidized" bed of solid particles may be supplied with the excessive amount of dried material, which will cause its conversion from the "fluidized" bed to the solid bed and its further blocking. On the other hand a decrease in the amount of the suspension dried leads either to an increase in the amount of the heat-carrier supplied to the "fluidized" bed of dried material solid particles, or to an increase in the dust ejection from the apparatus, which is also an undesirable phenomena.
It should be also noted that due to the above-mentioned reasons a certain ratio of the amounts of the heat-carrier supplied to the suspension spraying zone and to the "fluidized" bed of dried material solid particles is to be closely observed. As a consequence, apparatus aerodynamic and thermal conditions as well as control of these conditions are made more complex.
Furthermore, unloading of the finished product from the apparatus through the lower portion of the "fluidized" bed of dried material solid particles may result in the penetration of its fine particles into the finished product since an increase in the concentration of solid particles in the "fluidized" bed above the permissible may cause disruption of the "fluidized" bed with the penetration of dried material solid particles into the finished product. It should be also noted that upon moving in the "fluidized" bed dried material solid particles undergo abrasion, which leads to a lengthening of the granulation process and an increase in the dust ejection from the apparatus.
The above-described method and apparatus are also characterized by that the thermal treatment of the material by a high-temperature heat-carrier involves much difficulty since the heating of some materials and their dissociations may cause the formation of agglomerates (for example, in treatment of cement-raw granules), which leads to the clogging of the "fluidized" bed of dried material solid particles. And in case of the formation of a liquid phase on the surface of the granules of the thermally treated material the possibility of using the "fluidized" bed of solid particles is completely exluded due to the formation of large-size agglomerates. Thus, this prior art method and apparatus are characterized by that they do not allow to carry out thermal treatment of various materials and can be used only for suspension drying.
Furthermore, the method and apparatus under consideration do not enable to perform treatment of the suspension with varying capacities at a constant specific fuel consumption rate. The retention of the specific fuel consumption rate at a constant level presupposes that upon a 5 to 7-fold decrease in the amount of the suspension treated the total heat-carrier consumption should be also decreased just as much. In this case, however, a decrease in the amount of the heat-carrier supplied in a downcoming flow also the "fluidized" bed of dried material solid particles results in the disruption of aerodynamic operating conditions of the process and apparatus and, therefore, the treatment of the material becomes impossible.
It should be also noted that the prior art method envisages drying of the material in a direct current with the heat-carrier, which impairs the intensity of heat- and mass-transfer processes.